1. Field of the Invention
This invention relates to color cathode ray picture tubes, and is addressed specifically to a means and process providing for a color cathode ray tube having a tensed foil slit mask.
Color cathode ray tubes for television and computer displays universally employ an apertured shadow mask for ensuring that electrons from each of the three electron guns strike only areas capable of emitting light of the appropriate color. Two types of shadow masks are in common use: dot masks, with substantially circular apertures, are primarily used in computer display applications; slot masks, with parallel elongated apertures, are generally preferred in television receivers. Several well-known factors make the slot mask somewhat more economical in the television environment.
Two different forms of the slot mask can be found in conventional color tubes. In the most widely used form depicted in FIG. 1a, the slots are bridged by tie bars at frequent intervals, as indicated. Typically, the spacing between two tie bars in a given slot is of the same order as the center-to-center spacing between the two metal strips which form the slot. Due to the high density of tie bars, such a mask has very substantial mechanical strength in the transverse direction, i.e. at right angles to the major axis of the slots. Such strength is essential because the mask, after passing through the photo-etching processs which generates the slot-and-tie bar pattern, is formed into a dome shape in order to match the curved faceplate. During the forming process, the mask is stressed beyond its elastic limit, and it is essential that the tie bars do not break.
A second well known form of the slot mask (sometimes called a "slit" mask) uses no tie bars. The etched mask, essentially a parallel array of narrow strips held together only at the ends, is stretched over a strong, specially shaped frame so that the tensioned strips form a sector of a cylindrical surface (see FIG. 1b). The tension ensures that all strips remain straight. This design has the disadvantage that each strip is capable of vibrating independently, with very little damping. Conventionally, this deficiency is remedied by stretching one or several small diameter wires or fibers around the cylindrical surface, lightly touching all strips. (See U.S. Pat. No. 3,683,063.)
Both forms of the mask are generally made of sheet steel or similar material 0.005 to 0.010 inches thick, with the greater thicknesses used in larger tubes.
Recently, color cathode ray tubes were introduced in which the faceplate is not curved at all but is ground flat. The shadow mask in these tubes, referred to as tension mask tubes, is made of steel foil only 0.001 inch thick and held under high mechanical tension, amounting to a substantial fraction (typically more than 50%) of the elastic limit of the mask material. Dot masks (masks with circular holes) of this type, primarily intended for computer displays, may be put under uniform tension about their entire circumference. They are then welded, e.g. by laser welding, to four rail-like support structures surrounding the display area.
The tension mask tube offers a number of advantages over the conventional color tube with a curved faceplate and correspondingly curved mask. One important advantage lies in the fact that the photoetched mask is never stressed beyond its elastic limit. Therefore, masks made from the same master are alike and remain alike to a high degree of accuracy. Because the faceplate is also flat, the screen, i.e., the grille (black matrix) and the phosphor pattern can be deposited on the faceplate by a variety of printing processes such as offset printing or screen printing, thereby circumventing the cumbersome and costly photolithographic processes used in the manufacture of conventional color tubes. Screens made this way from a common master are also alike to a high degree of accuracy. Any mask may, therefore, be mated to any screen, for example by the process described in referent copending application Ser. No. 223,475, assigned to the assignee of this invention. For use in a television receiver, it may be desired to substitute a slot mask for the dot mask in a tension mask tube. The question then arises as to what the structure of the slot mask should be to achieve the desired performance at minimum cost. For example, it is possible to substitute a foil mask etched with the slot-and-tie bar pattern shown in FIG. 1a for the dot mask in the manufacturing process just described. Of course, the etched mask need not be formed into a dome shape but is allowed to remain flat. However, when the mask is stretched, care must be taken to apply just the right amount of force in the transverse direction, generally much less than the force applied parallel to the strips. Preferably this is done by observing the transverse displacements and feeding the information so obtained back to the force generating means, as taught in the above-mentioned '475 application. To lock the stretched mask in place, four support structures surrounding the display area are required just as in the case of the dot mask.
A "slot mask," as the term is used herein, has a dome shape. In its intended use and function, it is very similar to a "dot mask" but with elongated holes, the mask having enough structural strength in all directions to be metal-formed to a self-sustaining, three-dimensional curvature. A "slit mask," as the term is used herein, comprises parallel strips which have no interconnection.
Since in the manufacturing process of a tension mask tube the mask remains flat, it would appear that the tie bars shown in FIG. 1a are not needed. A foil mask may be etched in accordance with the pattern shown in FIG. 1b and stretched over just two support structures so as to put all strips under the desired tension. However, the fact that the foil is so thin makes it difficult to handle; it must be kept in mind that the bending stiffness of a strip is proportional to the cube of its thickness, so that a strip 0.001 inch thick is 125 times as flexible as a strip 0.005 inch in thickness. Therefore, if a foil mask patterned like the mask shown in FIG. 1b is to be used in production, it becomes necessary to resort to special handling techniques which increase the cost. In addition, the method of vibration damping based on stretching wires or fibers across the tensed mask is not as effective on a flat surface as it is on a cylindrical surface.
Thus it is evident that niether of the two known forms of the slot mask structure is fully satisfactory for use in a tension mask tube. The version derived from FIG. 1a requires four support structures in each tube, and to ensure interchangeability, an elaborate servo system is needed to control transverse displacement. The version derived from FIG. 1b requires only two support structures in each tube, but cost is increased by the need for special techniques to handle the mask, and there is a potential problem with insufficient vibration damping.
3. Other Prior Art
______________________________________ U.S. Pat. Nos. ______________________________________ 2,813,213 to Cramer et al 3,994,867 to Kaplan 2,842,696 to Fischer-Colbrie 4,100,451 to Palac 2,905,845 to Vincent 4,686,416 to Strauss 3,638,063 to Tachikawa 4,495,437 to Kume et al 3,894,321 to Moore 4,695,761 to Fendley 3,989,524 to Palac British Patent - GB 2 052 148 A to Sony ______________________________________
A journal article
"Improvements in the RCA Three-Beam Shadow Mask Color Kinescope," by Grimes et al. The IRE, Jan. 1954. Dec. Class. R583.6.1.